Viagra (sildenafil citrate) is a medicine used to treat the physical problem of erectile dysfunction (impotence) in men. It can help a majority of men with erectile dysfunction get and keep an erection when they become sexually excited. Simply taking Viagra will not make you have an erection. Unlike previously available drugs (like Yohimbine) Viagra does not causes erections directly but affects the response to sexual stimulation. Its not an aphrodisiac or a hormone.